Swimmers have been warned to take care at Christchurch beaches after hundreds of bluebottle jellyfish washed up on Spencer Beach.

Una Cunningham said she saw hundreds, if not thousands, of the jellyfish on Spencer Beach, just north of Christchurch, on Christmas Eve.

"We had never seen any before there, and we've been there fairly regularly since March," she said.

"The beach at the Spencer Park car park had a line of them and loads of seaweed as far as we could see going north and south. There would have been thousands of them on the beach."

Cunningham said she and her family did not go onto the beach as they did not want to walk among the jellyfish, which were blocking off the harder sand near the water.

Surf Life Saving New Zealand southern club development officer Henry Lawson said the jellyfish had been spotted "up and down the coast" over the last week, but no injuries had been reported to life guards.

The jellyfish tended to get stuck on Canterbury beaches during high tide, and came to the region as the ocean water heated up during summer.

"There's generally always some. Some years we can have thousands. There's been years where there's just been a big strip of blue right up the coast."

Lawson said he was not sure how many jellyfish had appeared over the last week, but they did not cause "a huge amount of trouble".

"It definitely hurts [to be stung]. It's not a pleasant experience, but a little bit of treatment and it's all fine."

Bluebottle jellyfish have small bodies, but come with long trailing tentacles which hold a sting.

The Ministry of Health advises those who have been stung by a bluebottle jellyfish to immerse the stung area in warm water (45 degrees Celsius) for 20 minutes.

Vinegar and fresh water should not be used for bluebottle stings.

Those who are allergic to bee stings should be particularly wary of bluebottles.